1. Field of the Invention
The present invention relates generally to asynchronous WCDMA (Wideband Code Division Multiple Access) communications, and in particular, to a method and an apparatus for efficiently controlling uplink (UL) traffic rates, performing UL rate scheduling accompanied by fast rate ramping, and transmitting rate assignment information.
2. Description of the Related Art
UMTS (Universal Mobile Telecommunication Service), one of the 3rd generation mobile communication systems implements WCDMA, based on the European mobile communication system, GSM (Global System for Mobile communication). The UMTS system provides a uniform service that transmits packetized text, digital voice and video, and multimedia data at or above 2 Mbps to mobile subscribers or computer users around the world. With the introduction of the concept of virtual access, UMTS allows access to any end point in a network all the time. The virtual access refers to packet-switched access using a packet protocol like IP (Internet Protocol).
FIG. 1 illustrates the configuration of a UTRAN (UMTS Terrestrial Radio Access Network).
Referring to FIG. 1, a UTRAN 12 comprises RNCs (Radio Network Controllers) 16a and 16b and a plurality of Node Bs 18a, 18b, 18c and 18d. The UTRAN 12 connects a UE 20 to a core network (CN) 10. A plurality of cells may underlie the Node Bs 18a to 18d. The RNC 16a controls the Node Bs 18a and 18b, and the RNC 16b controls the Node Bs 18c and 18d. The Node Bs 18a to 18d in turn control their underlying cells. An RNC, and Node Bs and cells under the control of the RNC are collectively called an RNS (Radio Network Subsystem).
The RNCs 16a and 16b assign or manage the radio resources of the Node Bs 18a to 18d within their coverage areas. The Node Bs 18a to 18d provide radio resources. Radio resources are configured on a cell basis, and the radio resources provided by the Node Bs 18a to 18d are those of their managed cells. The UE 20 establishes a radio channel using radio resources provided by a particular cell under a particular Node B and communicates on the radio channel. From the UE's perspective, discrimination between a Node B and a cell is meaningless. The UE 20 only recognizes physical channels established on a cell basis. Therefore, the terms Node B and cell are interchangeably used herein.
A Uu interface is defined between a UE and an RNC. The hierarchical protocol architecture of the Uu interface is illustrated in detail in FIG. 2. The Uu interface is divided into a control plane (C-plane) for exchanging control signals between the UE and the RNC and a user plane (U-plane) for transmitting actual data.
Referring to FIG. 2, C-plane signaling 30 is processed through an RRC (Radio Resource Control) layer 34, an RLC (Radio Link Control) layer 40, a MAC (Medium Access Control) layer 42, and a PHY (PHYsical) layer 44. U-plane information 32 is processed through a PDCP (Packet Data Control Protocol) layer 36, a BMC (Broadcast/Multicast Control) layer 38, the RLC layer 40, the MAC layer 42, and the PHY layer 44. The PHY layer 44 is defined in each cell, and the MAC layer 42 through the RRC layer 34 are defined in each RNC.
The PHY layer 44 provides an information delivery service using radio transfer technology. The information delivery service corresponds to layer 1 (L1) in an OSI (Open Systems Interconnection) model. The PHY layer 44 is connected to the MAC layer 42 via transport channels. The mapping relationship between the transport channels and physical channels is determined according to how data is processed in the PHY layer 44.
The MAC layer 42 and the RLC layer 40 of layer 2 (L2) are connected via logical channels. The MAC layer 42 delivers data received from the RLC layer 40 on logical channels to the PHY layer 44 on appropriate transport channels. It also delivers data received from the PHY layer 44 on transport channels to the RLC layer 40 on appropriate logical channels. The MAC layer 42 inserts additional information into the data received on logical channels or transport channels or performs an appropriate operation by interpreting inserted additional information, and controls the random access. While separately not shown, a U-plane-related part of the MAC layer 42 is referred to as a MAC_d and its C-plane-related part is referred to as a MAC-c.
The RLC layer 40 controls the establishment and release of the logical channels. The RLC layer 40 operates in one of an acknowledged mode (AM), an unacknowledged mode (UM), and a transparent mode (TM). Typically, the RLC layer 40 segments or concatenates SDUs (Service Data Units) received from an upper layer to an appropriate size and corrects errors.
The PDCP layer 36 is an upper layer of the RLC layer 40 on the U-plane. The PDCP layer 36 is responsible for compression and decompression of the header of the data in the form of an IP packet, and also controls the lossless data delivery when a change in an RNC providing service to a particular UE occurs due to the UE's mobility.
The characteristics of the transport channels that connect the PHY layer 44 to the upper layers depend on TF (Transport Format) that defines PHY layer processing involving convolutional channel encoding, interleaving, and service-specific rate matching.
The UMTS system uses an E-DCH or EUDCH (Enhanced Uplink Dedicated Channel) to more efficiently transmit packet data from UEs on the UL. To support high-speed data transmission more stably than a DCH (Dedicated Channel) used for general data transmission, the E-DCH adopts AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission request), and Node B controlled scheduling.
FIG. 3 conceptually illustrates data transmission on the E-DCH via radio links. Referring to FIG. 3, reference numeral 100 denotes a Node B supporting the E-DCH and reference numerals 101 to 104 denote UEs that transmit over the E-DCH. The Node B 100 detects the channel statuses of the UEs 101 to 104 using the E-DCH and schedules their UL data transmissions based on the channel statuses. The scheduling is performed such that a noise rise measurement does not exceed a target noise rise in the Node B, in order to increase the total system performance. Hence, the Node B 100 assigns a low data rate to the remote UE 104 and a high data rate to the nearby UE 101.
FIG. 4 is a diagram illustrating a signal flow for E-DCH transmission and reception. Referring to FIG. 4, a Node B and a UE establish an E-DCH in step 202. Step 202 involves the transmission of messages on dedicated transport channels. Then, the UE transmits scheduling information to the Node B in step 204. The scheduling information may contain UL channel information, that is, the transmit power and power margin of the UE, and the amount of buffered data to transmit to the Node B.
In step 206, the Node B monitors the scheduling information. When determining to allow the UE to transmit UL packets, the Node B transmits Scheduling Assignment information to the UE in step 208. The Scheduling Assignment information contains rate assignment information including an allowed data rate and timing.
The UE determines in step 210 the TF of the E-DCH based on the Scheduling Assignment information. In steps 212 and 214, the UE notifies at the same time the Node B of the TF and transmits UL packet data on the E-DCH. In step 216, the Node B determines if the TF information and the packet data have errors. In the presence of errors, the Node B transmits an NACK (Non-Acknowledgement) signal to the UE in step 218. In the absence of errors, the Node B transmits an ACK (Acknowledgement) signal to the UE in step 218. In the latter case, the packet data transmission is completed and thus the UE transmits new packet data to the Node B on the E-DCH. In the former case, the UE retransmits the same packet data to the Node B on the E-DCH.
Many scheduling methods are available for the above-described UL packet transmission. With reference to FIG. 5, one of the UL scheduling methods, rate scheduling will be described.
FIG. 5 illustrates the transmission of UL/DL (Uplink/Downlink) control information for rate scheduling, and UL rates controlled through the rate scheduling.
Referring to FIG. 5, a UE 304 transmits a Rate Request 308 and an E-DCH packet 310 to a Node B 302. The Node B 302 then generates and transmits a Rate Grant 306 indicating an allowed rate to the UE 304 after UL scheduling. Both the UE 304 and the Node B 302 are provided with a preset rate table. The rate table lists a plurality of available rates corresponding to their levels.
The UE 304 checks the amount of buffered UL data and an available power margin, and sends a rate up or rate down request for the E-DCH to the Node B 302 by the Rate Request 308. The Node B 302 determines whether to increase, decrease, or maintain the rate of the UE 304, taking into account rate requests from other UEs under the control of the Node B 302 as well as the rate request from the UE 304, and notifies the UE 304 of the determination result by the Rate Grant information 306.
More specifically, the UE 304 requests a rate up in an interval 312 by a Rate Request 314. Upon receipt of the Rate Request 314, the Node B 302 commands the UE 304 to increase its rate by a Rate Grant 318 in an interval 316, after scheduling. Thus, in an interval 322, the UE 304 transmits a UL packet at rate 11 one level higher than rate 10 used in an interval 320.
The above rate scheduling allows only a one-level rate change at a time. If the UE transmits data at a very low rate and wants to increase the rate by a plurality of levels, the Node B must transmit as many scheduling commands, for the rate increase. In this sense, the conventional rate scheduling is inflexible in rate change and causes a long time delay in achieving a desired rate by the UE.